The invention relates to the field of the production of aluminium by igneous electrolysis and, more particularly, to the management of certain operating stages of such installations.
More particularly, it relates to a unit for the collection of solid parts, in suspension or not in suspension in the electrolysis bath and, in particular, of blocks of anodes unhooked from the stem on which they are fixed, but also for the cleaning and calibration of the orifice left vacant by the replaced anode.
In a known manner, the production of aluminium by igneous electrolysis makes use of the electrolysis of the aluminium in a bath of molten cryolite, according to the reaction:
Al2O3 +2e +Na3[AlF6] xe2x86x922 Al +3O2 +3C +3CO2 +F
Aluminium cryolite oxide
This reaction uses a melting bath containing a mixture of cryolite and aluminium oxide and the temperature of which is generally greater than 800xc2x0 C. In consideration of the energies used and in order to limit as far as possible the losses inherent in the restart phases, installations which use this technology generally operate continuously at the level of series of aluminium pots, the number and the dimensions of which are a function, on the one hand, of the available amperage of the continuous electric current feeding the pots and, on the other hand, of the desired volume of production.
Normally, it is advisable to proceed to replace the different anodes, most often constructed in carbon, at the level of each of the pots, without, for all that, stopping the electrolytic reaction.
By virtue of the process used, namely igneous electrolysis, a hard crust of fluorinated cryolite and aluminium oxide is formed at the upper surface of the bath, this crust having the benefit of preserving the heat within the bath and thus of constituting a heat-insulating shell. Consequently, the extraction of spent anodes from the bath requires, in first place, the rupture of this crust. This rupture gives rise to the formation of fragments or solid parts floating on the surface or in suspension in the electrolysis bath and which need to be collected by means of a tool traditionally referred to as a xe2x80x9cshovelxe2x80x9d.
This shovel has also to prepare the space and the conditions necessary for the introduction of a new anode instead of and in place of the replaceable spent anode. This preparation also consists in calibrating the space needed for the installation of a new anode of this kind. In fact, the erosion of the generally rectangular section of the carbon constituting the anode actually brings about the diminution of the said section relative to a new anode, particularly owing to the progressive disappearance of the angular parts, which become rounded.
This transformation is inherent in the consumption of the carbon constituting the anodes, by virtue of the electrolytic process used. The space which is thus released between the blocks of carbon constituting the anodes is occupied by the bath, which surface-solidifies and forms the above-mentioned crust, the rupture of which is necessary to allow, on the one hand, the withdrawal of the spent anode and, on the other hand, the installation of a new anode. Thus, when the spent anode is to be removed, the crusts adhering thereto are previously shattered by means of an appropriate tool, known as a crust pick, and larger or smaller fragments of crust fall into the bath, float to the surface thereof and hence prevent the installation of a new anode.
Moreover, more or less voluminous carbon blocks can also float in the bath following the actual pull-out operation of the anode.
Finally, on the surface of the cathode, that is to say the base of the pot, slimes originating from the electrolytic bath are deposited, the removal of which is desirable since they increase electrical resistance and hence reduce the pot production yield.
The shovel therefore fulfils firstly a collection function, secondly, a cleaning function and, finally, a calibration function. In order to ensure these different functions, it is known to use hinged buckets, the actuation of which causes their lips disposed at the level of their free edge to come together and hence to produce a shovel. Now, these buckets are hinge-mounted on a chassis, such that the actuation of the connecting rod assembly traditionally used to ensure the closure of the buckets causes the two lips to converge, describing an arc of a circle as they do so, the concavity of which is directed upwards.
Since the lips are intended to ensure a gentle dredging of the base of the pot, that is to say of the cathode, and taking into account, moreover, the weight of the buckets, their actuation mechanism and, in general terms, the weight of the shovel, this circular movement is liable to damage this cathode and, hence, the pot itself.
Consequently, and in order to avoid such damage, the vertical positioning of traditional shovels is chosen such that, when the bucket is closed, the lips lie flush with the bottom of the cathode. In other words, the lips can only ensure an efficient dredging of the base of the pot at the sole level of their closing zone, so much so that in the long run it is necessary to drain the pots in order to ensure effective cleaning of the latter, therefore implying a stoppage of the installation, failing which a significant fall occurs in the yield from the said installations.
Moreover, the shovel being intended to work in a hostile and aggressive environment (temperature of the electrolytic bath close to 800xc2x0 C., very acidic fluorine-containing fumes, etc.), it is not feasible to resort to sensors for managing the vertical travel of the shovel as a function of the degree of closure of the buckets.
The object of the invention is to eliminate these drawbacks. It relates to a unit for the collection, cleaning and calibration of electrolysis pots and, more precisely, of the anodic orifices, as the anodes are replaced, by means of which unit a calibration of these orifices, the collection of waste inherent in the rupture of the crust which forms at the surface of the bath and, furthermore, the cleaning of the base of the pot by dredging can be simultaneously ensured.
This unit comprises two buckets hinge-mounted at the level of the lower end of a chassis movable in vertical displacement, the free edge of each of the said buckets being liable to be bestowed with a circular movement by means of a closing and opening connecting rod assembly.
According to the invention, the unit comprises a first chassis, joined to a vertical displacement mast, referred to as the shovel-bearing chassis, and a second chassis, referred to as the buckets-supporting chassis, mechanically linked to the said shovel-bearing chassis and liable to displacement relative to the latter, the said buckets-supporting chassis integrating the connecting rod assembly for closing and opening the buckets, the said buckets being hinge-mounted at the lower end of this chassis.
In other words, the invention consists in adding to the traditional support of the actual tools constituting the shovel a chassis linked to the said support but nevertheless movable relative to the latter, this mobility being intended to allow the execution of a movement simultaneous with the buckets-closing operation and appropriate for compensating the circular movement of the lips of the said buckets in such a way as to impart to the lips a substantially rectilinear movement, parallel to the plane of the base of the pot.
The shovel is thereby endowed with one of the functions for which it is required, namely the operation involving dredging of the base of the pot.
According to one characteristic of the invention, the closing connecting rod assembly of the buckets comprises a force-transmitting rod, one of whose ends is hinge-mounted directly or indirectly on the said buckets so as to impart to their free lower edge a circular movement as the shovel is closed and whose other end is hinge-mounted on a rotary connecting rod, itself hinge-mounted on the buckets-supporting chassis, the said rotary connecting rod being mechanically linked to the shovel-bearing chassis by means of a compensating rod and being set into rotation, moreover, by means of an actuating jack, the point of application of which is integral with the buckets-supporting chassis.
According to an advantageous characteristic of the invention, the actuating jack is made undersized by means of a calibrated spring, resting on the shovel-bearing chassis and liable to absorb, at the level of the body of the jack, a part of the forces and, in particular, the compensation of the mass constituted by the movable aggregate, that is to say the closing connecting rod assembly of the buckets and the buckets themselves. This spring can be replaced by an equivalent member, such as, for example, a guided counterweight connected to a cable and a return pulley.
The manner in which the invention can be realized and the advantages which follow therefrom will be more clearly apparent from the following illustrative embodiment, provided on an indicative and non-limiting basis in support of the appended figures.